A thermoelectric device is capable of generating electricity if two ends of the thermoelectric device are held at different temperatures. When two dissimilar metals (conductors) or semiconductors having different Seebeck potential or Fermi energy levels are in contact at each end, a voltage is obtained if the ends are at different temperatures (i.e., the Seebeck effect). Conversely, an applied electrical current can induce a temperature differential between the two ends due to the Peltier effect. The Peltier effect causes absorption or liberation of heat when current flows across the junction of two dissimilar materials. As electrons flow from a p-type semiconductor to an n-type semiconductor an energy gap or “hurdle” is traversed. Thermal energy is absorbed as electrons overcome this energy hurdle, and this junction is cooled. Conversely, as electrons flow from an n-type semiconductor to a p-type semiconductor, electrons “fall” down the energy gap and thus release heat. This release will locally heat the junction.
Thus, a thermoelectric device can be a cooler or a heat pump which transfers heat by electric current. The principles of thermoelectricity are utilized in power generation, thermocouples, and refrigeration. The efficiency of a thermoelectric device can be expressed in terms of a figure of merit (ZT). In order for a material to be efficient for thermoelectric power conversion, it is important to allow charge carriers to diffuse easily across multiple Peltier couples while maintaining a temperature gradient. That is, there must be a relatively high value for the Seebeck coefficient (S), a high electrical conductivity (a), and a low thermal conductivity (K). Current designs of commercially available thermoelectric devices have efficiencies too low to warrant widespread cooling application. However, improvements in the thermoelectric material properties and thermoelectric device design are expected to provide thermoelectric devices with enhanced thermal performance. These devices will be better suited for power generation, cooling, and temperature control applications.
Typically, a thermoelectric device contains p-type and n-type semiconducting materials sandwiched between two ceramic plates, for example an upper and lower faceplate or carrier plate. The faceplates typically have high electrical resistivity and low thermal conductivity. Situated between the faceplates are a number of Peltier couples, formed by joining p-type and n-type semiconductor elements. These couples can be arranged in a two-dimensional array, thermally in parallel, and connected by conductors (braze, solder, and the like) so as to be electrically in series. Typically, a device being cooled is placed in thermal contact with the cold faceplate, and a heat sink is placed in contact with the hot faceplate.
Accordingly, a thermoelectric device technology typically uses a bipolar, p-n couple with two temperature zones as shown in FIG. 1. FIG. 1 depicts a conventional bipolar p-n couple 10 having two legs 10a and 10b of opposite conductivity type. As shown, the bipolar p-n couple 10 is configured with a polarity for cooling at heat source 12. The legs 10 and 10b are connected electrically in series and thermally in parallel such that current flow serially through the legs 10a and 10b carries heat to heat sink 14 where the heat is dissipated from the thermoelectric device. Consequently, the bipolar p-n couple 10 utilizes two temperature zones connected respectively to the heat source 12 and the heat sink 14. Fins 16, as shown, are frequently utilized on either the heat source 12 or the heat sink 16, if needed.
However, the utilization of different n and p-type materials adds complications to the manufacturing process and frequently costs efficiency of the fabricated thermoelectric device, as the thermoelectric performance of one of the n and p-type materials is typically lower the thermoelectric performance of the other of the opposite type.